Button needs replacing. Until then, you can use the accessability option in settings. Turn it on, then you can use the grey floating accessability dot to lock and unlock your phone. If you need to power off, press and hold the lock icon on accessability menu, and the "slide to power off" will show up. To power on again, just plug phone in and it will come back on after a few seconds.
Under settings, accessability, turn on assistive touch. You'll have a grey dot floating around on your screen. When you touch the dot, it brings up the full "menu." Touch "device" then a lock button option will show. Press and hold the lock icon. You'll then have the "slide to power off" option. Turn off. Give it a couple minutes. Don't panic, even without the working power button to turn it back on, it's still doable! Just plug the phone in and it'll power right on.